Does a dehumidifier in the bedroom help protect a silk press?

Yes, in genuinely humid climates it can help meaningfully by lowering ambient moisture in the room overall. It works on the air side of the equation; the pillowcase works on the fabric-contact side — together they cover more of the exposure than either alone.

Humidity & Silk Press

Why Your Silk Press Frizzes on Humid Nights Even Without Rain

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Hair Doesn't Need Rain to Absorb Moisture — Air Is Enough

Hair is hygroscopic, meaning it pulls moisture directly from humid air, not just from direct water contact. On a muggy night, ambient humidity alone can be enough to swell the hair shaft and revert a smoothed cuticle, which is why a press can frizz even when nothing actually touched it.

What You Sleep On Adds or Subtracts from That Exposure

A cotton pillowcase already holding moisture from a humid room offers no barrier — if anything, it stays damp against your hair for hours. A non-absorbent silk surface doesn't hold that ambient moisture against your scalp and hairline the same way, reducing one layer of exposure even on a humid night.

      "It's tempting to blame frizz on rain or the weather forecast. But ambient humidity in a bedroom does the same damage silently, every single night, regardless of what's happening outside.
      "
    

Pillowcase	Holds Ambient Moisture	Adds to Humidity Exposure
Cotton	Yes	Yes
Polyester Satin	Partial	Some
Silkhouse Silk	No	No

Silkhouse silk pillowcase against humidity frizz

You Can't Control the Weather, but You Can Control the Fabric

Humid nights aren't avoidable in most climates. What touches your hair while you sleep through them is. Silkhouse pillowcases are 100% mulberry silk, non-absorbent by nature.